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        <h1>PrettyProlog - the Event Listeners</h1>
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	  This module contains classes that are either events - holders for various pieces of information associated with a certain action or change in an object's state - or event listeners, i.e. classes whose instances can be registered with other objects in order to receive the events mentioned above. In particular, event listeners can be associated with the Engine, the Theory, and the Stack. Engine listeners can receive events such as the finding of a solution, or the call of some method that controls the solving process. Theory listeners react to the addition or removal of clauses from the Theory, and to the selection of a clause via the nextClause() method. The Stack listeners get notified of the events of push, pop, cut (when a Frame is removed as the effect of a cut) and clear (when the Stack is emptied).
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